Ethylene oxidation in the presence of iridium metal

ABSTRACT

ETHYLENE IS OXIDIZED WITH OXYGEN TO ACETIC ACID WHICH IS SUBSTANTIALLY FREE OF OTHER OXYGENATED ORGANIC PRODUCTS BY OXIDATION OF THE ETHYLENE IN THE PRESENCE OF IRIDIUM METAL.

United States Patent O- 3,641,139 ETHYLENE OXIDATION IN THE PRESENCE OF IRIDIUM METAL Noel W. Cant and William K. Hall, Pittsburgh, Pa., as-

signors to Gulf Research & Development Company, Pittsburgh, Pa. No Drawing. Filed Nov. 25, 1968, Ser. No. 778,806 Int. Cl. C07c 53/08 US. Cl. 260533 R 8 Claims ABSTRACT OF THE DISCLOSURE Ethylene is oxidized with oxygen to acetic acid which is substantially free of other oxygenated organic products by oxidation of the ethylene in the presence of iridium metal.

This invention relates to a process for oxidizing ethylene to obtain acetic acid which is substantially free of other oxygenated organic products.

It has been found that the oxidation of ethylene can be made more selective to the formation of acetic acid which is substantially free of other oxygenated organic products by oxidizing ethylene in the presence of iridium metal.

In order to obtain good selectivity to the desired acetic acid, the ethylene together with a gas containing free molecular oxygen are preferably contacted with the iridium metal at a temperature within the range of about 50 to about 250 C., more preferably within a range of about 70 to about 150 C. One suitable method of contacting is to pass a mixture of ethylene and a gas containing free molecular oxygen, such as air, or a mixture of oxygen and an inert gas, such as helium, through a bed of iridium metal either supportedor unsupported. Since the reaction is exothermic, means must be provided to control the temperature of the reaction within the limits defined above. Below the lower temperature limit defined above, the reaction rate becomes too slow to be economically feasible, Whereas at temperatures above the upper limit, the yield of the desired acetic acid decreases with, the concurrent production of excessive amounts of by-product water and carbon dioxide due to secondary reactions. The reaction temperature can be controlled by any suitable means, and one method of at least partially controlling the temperature is to dilute the iridium metal by distending it on a suitable support material such as' silica, magnesia, alumina, thoria or mixtures thereof. The iridium metal can, of course, be used unsupported and when this is done the metal can be in any suitable form, such as sponge form. When the iridium is distended on a support, the surface area of the support is not critical and can suitably be between 0.1 and 600 square meters per gram. In addition to the above named supports, materials such as carbon, kiesulguhr, pumice, the natural clays, mullite and alundum are also suitable. When the iridium metal is supported, suitable amounts f iridium metal are from 0.2 to 30 weight percent of the total catalyst with preferred amounts from 1 to 10 weight percent and the more preferred amounts from 1 to weight percent of the total catalyst.

Thhe method of preparing the supported or unsupported catalysts is not critical. Suitable methods of preparing the supported catalysts, for example, include the method of incipient wetness using aqueous solutions of suitable iridium salts such as H IrCl followed by drying and reduction in hydrogen to obtain the metal.

Another method of controlling the temperature is to dilute the ethylene-oxygen mixture with an inert gas such as nitrogen or helium. Yet another method of controlling the reaction temperature is to pass the admixture of ethylene and oxygen over the iridium metal catalyst ice at very high space velocities. Suitable gaseous space velocities are within the range of about 1 to about 2000 volumes of ethylene measured at standard temperature and pressure per volume of catalyst per hour, and the preferred space velocities are from about 5 to about 200.

A total operating pressure of about one atmosphere is the desired operating pressure. Higher or lower pressures can be used; for example, a total pressure of from 0.5 to 15 atmospheres or more can suitably be employed.

The ratio of the partial pressure of oxygen to the partial pressure of ethylene can suitably be between 0.1 and 100 and is preferably between 0.4 and 15. The partial pressure of ethylene should be at least 0.1 p.s.i.a. and is preferably from 0.4 to 1.5 p.s.i.a. when the total pressure is atmospheric. Correspondingly higher partial pressures of ethylene would be employed at the correspondingly higher total operating pressures.

' The ethylene is oxidized in the presence of a gas containing free molecular oxygen. Pure oxygen can be used, but this creates problems of temperature control as noted above. It is preferred that the free molecular oxygen be diluted with an inert gas such as nitrogen or helium. The volume percent of free molecular oxygen in the gas containing it can suitably be between one and 100 and is preferably between one and 20. When ethylene is mixed with this gas containing free molecular oxygen the partial pressure of oxygen is usually between 0.5 and 15 p.s.i.a. and is preferably between one and four p.s.i.a.

The invention will be further described with reference to the following experimental work. In all of the examples to follow the following procedures was employed. A single pass flow system was used wherein a feed mixture of ethylene, oxygen and helium was passed through a bed (approximately two parts by volume of catalyst of a supported iridium catalyst) at a given temperature between 50 and 200 C. at a flow rate of between 2400 and 3000 volumes of total feed per hour. The contact time was about 2.3 seconds and the space velocity based on the total feed was about 1500 volumes of feed per volume of catalyst per hour. The ethylene space velocities for any particular run can be calculated by multiplying 1500 by the ratio of the ethylene partial pressure in millimeters to the total pressure of about 740 millimeters. The reaction products were cooled to -80 C. by indirect cooling to condense the reaction product in which acetic acid was observed. Trace amounts of acetaldehyde were also found. Water and CO were formed as by-products. By trace amounts is meant less than one volume percent of the acetic acid product so that the acetic acid was substantially free of other oxygenated organic products.

EXAMPLE 1 EXAMPLE 2 In the run for this example, the feed mixture was passed through a bed of a silica gel supported iridium catalyst containing 5 weight percent iridium. The silica gel support was a commercial Cab-O-Sil material from the Cabot Company. The catalyst was prepared by the method of incipient wetness by contacting the Cab-O-Sil with an aqueous solution of an appropriate amount of an iridium salt, i.e. H IrCl drying the material and reducing at 300 C. in hydrogen to convert the salt to metallic iridium. The results of this run are summarized on Table Ibelow.

Referring to Table I below, the percent selectivity to acetic acid was about the same (12 and 10 percent respectively for Examples 1 and 2) whether alumina or silica was used as the support. Trace amounts of acetaldehyde were also found.

rate, however, increases with an increase in ethylene partial pressure (compare Examples 11-13 so that a compromise ethylene pressure is required to balance the desired rate and selectivities.

Obviously, many modifications and variations of the invention as hereinabove can be made without departing from the spirit and the scope thereof, and such modifications and variations are intended to be included within the scope of this invention.

TABLE I Products of ethylene oxidation over supported iridium at a total pressure of about 740 mm.

Oxidation Percent rate 1 selec- Weight Oxygen Ethylene Tempervolumes tivlty 2 Example percent pressure pressure ature (02114) acetic No. Metal Support metal (mm.) (mm.) C.) min.- acid 1 Ir Gamma-A1203..- 4. 5 40 29 143 0. 13 12 2 Ir SiOz 5. 52 20 109 0. 07 10 1 The oxidation rate is defined as the rate at which ethylene is converted to all products in volumes (STP) per minute, e.g. if the ethylene is passed over the catalyst at 2 volumes per minute and it is found that 1.8 volumes per minute is recovered unchanged, then the oxidation rate is 2.0 minus 1.8, or 0.2 volumes per minute.

2 The percent selectivity is defined as the percent of the ethylene oxidized which is converted to acetic acid.

A series of runs was made by passing a feed mixture over the catalyst of Example 2 at varying temperatures and partial pressures of ethylene and oxygen to determine the eifect of these variables on reaction rate and selectivity to the production of acetic acid. The results are given on Table II below. The difference between the partial pressures of oxygen and ethylene and the operating pressure of one atmosphere was made up with helium.

Referring to Table II below, it can be seen that the selectivity is nearly independent of temperature and oxygen pressure over the range shown and is highest for the low ethylene pressures (compare Examples 13-15). The

TABLE II Dependence of rate of selectivity of ethylene oxidation over Ir/Si0 on process variables at total pressure of one atmosphere Oxidation,

Oxygen Ethylene rate partial partial volumes Percent Example Temp. pressure pressure (C2H4) Selec- No. 0.) (mm.) (mm.) min.- tivity e B See Table I footnotes.

We claim:

1. A process for the production of a product comprising acetic acid, which process comprises reacting ethylene with a gas containing free molecular oxygen in the contact presence of iridium metal.

2. A process according to claim 1 wherein the reaction occurs at a temperature from 50 to 250 C.

3. The process of claim 2 wherein the temperature in the reaction zone is maintained in the range of about to about C. and the space velocity is from 1 to 2000 volumes of ethylene measured at standard temperature and pressure per volume of catalyst per hour.

4. A process according to claim 2 wherein the ratio of the partial pressure of oxygen to the partial pressure of ethylene is between 0.1 to 100.

5. The process of claim 1 wherein the iridium metal is deposited on a support.

6. A process according to claim 5 wherein the reaction occurs at a temperature from 50 to 250 C., and the amount of iridium metal is from 0.2 to 30 weight percent of the total catalyst.

7. A process according to claim 6 wherein the support is gamma-alumina.

8. A process according to claim 6 wherein the support is silica.

References Cited UNITED STATES PATENTS 3,439,044 4/1969 Hirsch ct a1. 260-604 3,534,093 10/1970 Gerberich et al. 260-533 R LORRAINE A. WEINBERGER, Primary Examiner R. D. KELLY, Assistant Examiner 

